Device-to-device (D2D) communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite, such as WiFi Direct. These example systems operate in unlicensed spectrum.
Although the idea of enabling D2D communications as a means of relaying in cellular networks was proposed by some early works on ad hoc networks, the concept of allowing local D2D communications to (re)use cellular spectrum resources simultaneously with ongoing cellular traffic is relatively new. Because the non-orthogonal resource sharing between the cellular and the D2D layers has the potential of reuse gain and proximity gain, along with increased resource utilization, the concept of D2D communications underlying cellular networks has received considerable interest in recent years.
The Third Generation Partnership Project (3GPP) refers to Network Controlled D2D as “Proximity Services” or “ProSe,” and efforts aimed at integrated D2D functionality into the Long Term Evolution (LTE) specifications are underway. A ProSe Study Item (SI) recommended supporting D2D operation between wireless devices—referred to as user equipments or UEs by 3GPP—that are outside of network coverage, as well as operation between in-coverage and out-of-coverage wireless devices.
Specifically, in 3GPP LTE networks, such LTE Direct (D2D) communication can be used in commercial applications, such as cellular network offloading, proximity based social networking. D2D communications involving out-of-coverage operation are expected to be particularly important in so-called national security and public safety services (NSPS), such as in public safety situations in which first responders need to communicate with each other and with people in a disaster area. Both commercial and public safety applications are among the use cases discussed in the feasibility study performed by members of the 3rd-Generation Partnership Project (3GPP) and documented in the report “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Feasibility study for Proximity Services (ProSE),” 3GPP TR 22.803, v. 12.2.0 (June 2013), available at www.3gpp.org.
D2D communication entities using an LTE Direct link may reuse the same physical resource blocks (“PRBs,” the basic time-frequency resource in the LTE radio link) as used for cellular communications either in the downlink or in the uplink or both. The reuse of radio resources in a controlled fashion can lead to the increase of spectral efficiency at the expense of some increase of the intra-cell interference.
Typically, D2D communicating entities in an LTE-underlying scenario will use uplink (UL) resources, such as UL PRBs or UL time slots, but conceptually it is possible that D2D (LTE Direct) communications takes place in the cellular downlink (DL) spectrum or in DL time slots. For ease of presentation, in the present disclosure it is assumed that D2D links use uplink resources, such as uplink PRBs in a Frequency-Division Duplexing (FDD) LTE system, or uplink time slots in an a cellular Time-Division Duplexing (TDD) system, but the essential ideas disclosed herein may be readily applied to cases in which D2D communications take place in DL spectrum as well.
An important aspect of D2D communications is the concept of “discovery,” which deals with the techniques and procedures used by a D2D device to detect the proximity of other devices that are capable of and authorized to engage in D2D communications.
The 3GPP technical report titled “Study on architecture enhancements to support Proximity-based Services (ProSe),” briefly outlines several approaches to discovery, including an “LTE based solution for direct discovery.” (3GPP TR 23.703, sec. 6.1, v1.1.0 (January 2014).) As shown in this document, an “announcing UE,” i.e., a wireless device that wishes to make its services or need for services known to other devices, receives a so-called expression code from a ProSe function in or attached to the LTE network, e.g., in a ProSe server, and broadcasts the received expression code. Other wireless devices, referred to in 3GPP documentation as “monitoring UEs,” monitor for broadcasts from announcing UEs and compare any received expression code to one or more expression codes that they may have previously received, to detect the presence of announcing UEs they are interested in communicating with.
Although details of the exact discovery procedures to be carried out by 3GPP-compatible devices are still being developed, section 6.1.13 of the 3GPP technical report referenced above describes a network-controlled discovery procedure in which one or more “application IDs,” which can be viewed as examples of the “expression codes” discussed above, are allocated to announcing and monitoring UEs. These application codes have a limited life, and are associated with a validity timer that begins at the time the application codes are allocated. The announcing UE is allowed to broadcast a ProSe_Code that includes the application code until the validity timer expires. Monitoring UEs monitor the appropriate radio resources, “listening” for a ProSe_Code that includes an application code corresponding to an application code that has been allocated to it. Once the validity timer expires, the announcing UE must initiate a new request to the ProSe function to receive a new application code or codes.